Sulfuric acid bleaching of kraft pulp I: Bleaching of hardwood and softwood kraft pulps

To develop a new nonchlorine bleaching technology, hardwood and softwood kr aft pulps, before and after oxygen-alkali predelignification, were treated with dilute sulfuric acid solutions (pH 1.0-1.8) at 100 degrees C for Ih an d then extracted with aqueous sodium hydroxide at 70 degrees C for Ih. Hard wood kraft pulp was successfully bleached. The delignification selectivity was similar to that seen with oxygen-alkali bleaching; and it was greatly e nhanced by the addition of sodium nitrate and sodium nitrite. The sulfuric acid bleaching can replace the presently adopted oxygen and chlorine stages if the additives are allowed. This bleaching process was also effective fo r oxygen-bleached hardwood kraft pulp? but it was less effective for softwo od kraft pulp and oxygen-bleached softwood kraft pulp. The effectiveness of the addition of sodium nitrate and sodium nitrite was more apparent for so ftwood kraft pulp than for hardwood kraft pulp.